The present invention generally relates to a modulation and error-control system. More particularly, the present invention relates to a multirate modulation and eror-control system based on orthogonal bipolar spreading vectors that provides constant bandwidth and constant envelope signal.
In a digital satellite communication system, it is often desirable to support differing information transfer rates with a common modulation basis. A constant envelope (where the locus of the quadrature components in phase space of the modulation process is constrained to a circle) is often preferred to minimize spectral regrowth and other adverse effects that arise when variable envelope signals traverse a High Power Amplifier (HPA), such as a traveling wave tube.
One approach to providing differing information transfer rates is to combine individual carriers using Frequency Division Multiplexing (FDM) of several basic rate channels together with inverse multiplexing to distribute an aggregate rate stream among several lower rate transport paths. However, the fact that several carriers must be combined in FDM results, unavoidably, in amplitude fluctuations in the envelope of the combined signal which, in turn, may require additional back-off in the final microwave amplifier for the transmission to avoid intermodulation. Equivalently, a higher rated HPA may be required.
Another approach to providing differing information transfer rates is to use time multiplexing, based on Time Division Multiple Access (TDMA). In this method, all terminals are required to be able to transmit at a common burst rate and to be able to expand or contract their information transfer rate by actively transmitting during more or less of a repetitive time frame (i.e., in more or fewer time slots). This method (widely used in current satellite transmission) permits single access to the microwave amplifier so that constant envelope transmission is achieved. However, TDMA has the drawback that every terminal's HPA must be rated to support the burst rate. Since the burst rate may be much higher than the highest desired information transfer rate of a terminal, a cost penalty may be imposed on the terminal designer of lower rate terminals.
Depending on the available contrast (ratio of desired signal to undesired background) it may also be required to provide redundancy coding to achieve error control. In previous systems such error control is applied extrinsically by separate encoders and decoders.
These and other problems demonstrate that a need has long existed for a modulation and error-control system that is scalable to differing information transfer rates while providing a constant envelope and constant bandwidth. Such a system eliminates the need for the costly HPAs needed to support the burst rate in TDMA and additionally eliminates the amplitude fluctuations found in the envelope of FDM transmissions. Such a system also eliminates the need for separate error control apparatus.